AR15-pattern lower receivers that are configured to accept common pistol caliber magazines are well-known and increasing in popularity. A common caliber for such variant firearms is the 9×19 mm Parabellum, because of its wide-spread popularity as a pistol round. Likewise, due to the popularity of GLOCK® handguns and readily-available magazines for GLOCK® handguns in this caliber, it is a natural choice to configure these modified AR15-pattern receivers to accept these popular ammunition magazines.
In a semi-automatic firearm, the preference is for the design to include a feature that allows the bolt to be automatically held back in the open position after the last round of ammunition is fired from the magazine. When making a variant of an AR15-pattern firearm, it is preferred to use as many standard parts as possible, both because such parts are widely available at low cost and because the user is more likely to already have familiarity with the placement and operation of the various operational controls.
AR15-pattern lower receivers use a bolt catch mechanism that is not compatible with GLOCK®-pattern magazines without significant adaptation. Known prior art devices typically use a member formed from sheet metal in an attempt to transfer the actuation motion of the GLOCK® magazine follower to the bolt catch member mounted on the lower receiver. As illustrated in FIG. 1, these prior art devices rely purely on up and down motion exerted on a thin sheet metal finger to transfer the upward motion from the magazine follower to the bolt catch member. The prior art lower receiver 10 includes a cavity 12 positioned aft of the magazine well 14 to receive a vertically slidable bolt catch actuator 16 which extends forwardly to engage the magazine follower and lift a blocking portion 18 of the bolt catch member 20 when an ammunition magazine (not shown in FIG. 1) is inserted in the magazine well 14 and is empty. These mechanisms are notoriously unreliable due to the sheet metal's flexibility, unsuitable leverage points against an extended finger of the actuator 16, and the requirement for significant machining to the lower receiver 10.